A 30-year old American citizen who has been detained in Yemen for more than four years is missing, according to his lawyers.

Sharif Mobley has been in custody since 2010. He is accused of murdering a Yemeni guard who was monitoring him after he was captured in the capital, Sanaa, on terrorism charges that have since been dropped.

His abrupt disappearance comes ahead of a court hearing at which his lawyers planned to introduce what his lawyers say is evidence of US complicity in his capture and subsequent interrogation.

Mobley’s lawyers, who are with the UK-based human rights group Reprieve, last spoke to him at Sanaa’s Central Prison on 27 February. Weeks later, while attempting to set up another visit, the lawyers were told by Yemeni authorities that Mobley had been moved to an undisclosed location. They have not heard from him since.

On Wednesday, a representative of the Yemeni embassy in Washington told Mother Jones that Mobley was in Central Prison. A Reprieve spokeswoman, Katherine O’Shea, told the Guardian the organization’s lawyers visited all four acknowledged detention sites in Sana’a on Thursday – including Central Prison – and could not locate Mobley.

Reprieve, citing correspondence with an unnamed US Embassy official in Sanaa, said the Yemeni Interior Ministry has declined to tell US diplomats where Mobley is.

Mobley’s Reprieve lawyer, Cori Crider, said she considered her client to be a “disappeared” person.

“You ship someone to a detention site and refuse to tell his family, his lawyers, or his government where he is, you have disappeared him. I know the embassy spokesperson doesn't like it, but that is textbook disappearance,” Crider said.

The US Embassy referred all requests for comment to the State Department, where a consular affairs press officer, Jessica Wolf-Hudson, said that “due to privacy considerations, we are unable to provide further information” beyond saying that consular officers “strive to assist US citizens detained abroad whenever possible”.

The Yemeni embassy in Washington did not return a request for comment.

Mobley, who had lived in New Jersey, was initially suspected of terrorism, but Yemen has dropped those charges against him.

Weeks after so-called "underwear bomber" Umar Farouk Abdulmutallab attempted to blow up a plane heading for Detroit on Christmas Day 2009, eight armed men in Sanaa took Mobley at gunpoint, shooting him in the leg during his capture.

Reportedly, US investigators thought Mobley could lead them to Anwar al-Awlaki, the al-Qaida propagandist and American citizen who encouraged Abdulmutallab. The US killed al-Awlaki in a drone strike in 2011.

Mobley, whom friends in the US told the Washington Post had become radicalized, had lived in Yemen since 2008 with his wife and young daughter, studying Islam and Arabic.

After his capture, Mobley was held in secret and interrogated for weeks. Some of those interrogations included men who identified themselves to Mobley as representing the FBI and the Department of Defense, according to an FBI interrogation log acquired by Reprieve through the Freedom of Information Act.

“Mobley was medically cleared for questioning by his physicians. Mobley appeared to be receiving excellent medical care in a state of the art facility,” reads the heavily redacted log, dated 30 January 2010.

According to Reprieve, Mobley’s interrogators threatened to rape him and his wife.

Yemeni officials accuse Mobley of shooting two guards, killing one, at the hospital where he was recuperating from the wound in his leg in what they say was a March 2010 escape attempt. He has been imprisoned in Yemen since.

“This case has been one regrettable episode after the next. First Mobley gets seized and shot, then both the US and Yemen abused him in secret detention, and now we are having to defend him in court with authorities who see fit to disappear him for weeks at a time,” Crider said.